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<h3 class="section" id="Built_002din-Functions-to-Perform-Arithmetic-with-Overflow-Checking"><span>6.56 Built-in Functions to Perform Arithmetic with Overflow Checking<a class="copiable-link" href="#Built_002din-Functions-to-Perform-Arithmetic-with-Overflow-Checking"> &para;</a></span></h3>

<p>The following built-in functions allow performing simple arithmetic operations
together with checking whether the operations overflowed.
</p>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-_005f_005fbuiltin_005fadd_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_add_overflow</strong> <code class="def-code-arguments">(<var class="var">type1</var> <var class="var">a</var>, <var class="var">type2</var> <var class="var">b</var>, <var class="var">type3</var> *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fadd_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fsadd_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_sadd_overflow</strong> <code class="def-code-arguments">(int <var class="var">a</var>, int <var class="var">b</var>, int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fsadd_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fsaddl_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_saddl_overflow</strong> <code class="def-code-arguments">(long int <var class="var">a</var>, long int <var class="var">b</var>, long int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fsaddl_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fsaddll_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_saddll_overflow</strong> <code class="def-code-arguments">(long long int <var class="var">a</var>, long long int <var class="var">b</var>, long long int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fsaddll_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fuadd_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_uadd_overflow</strong> <code class="def-code-arguments">(unsigned int <var class="var">a</var>, unsigned int <var class="var">b</var>, unsigned int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fuadd_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fuaddl_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_uaddl_overflow</strong> <code class="def-code-arguments">(unsigned long int <var class="var">a</var>, unsigned long int <var class="var">b</var>, unsigned long int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fuaddl_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fuaddll_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_uaddll_overflow</strong> <code class="def-code-arguments">(unsigned long long int <var class="var">a</var>, unsigned long long int <var class="var">b</var>, unsigned long long int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fuaddll_005foverflow"> &para;</a></span></dt>
<dd>
<p>These built-in functions promote the first two operands into infinite precision signed
type and perform addition on those promoted operands.  The result is then
cast to the type the third pointer argument points to and stored there.
If the stored result is equal to the infinite precision result, the built-in
functions return <code class="code">false</code>, otherwise they return <code class="code">true</code>.  As the addition is
performed in infinite signed precision, these built-in functions have fully defined
behavior for all argument values.
</p>
<p>The first built-in function allows arbitrary integral types for operands and
the result type must be pointer to some integral type other than enumerated or
boolean type, the rest of the built-in functions have explicit integer types.
</p>
<p>The compiler will attempt to use hardware instructions to implement
these built-in functions where possible, like conditional jump on overflow
after addition, conditional jump on carry etc.
</p>
</dd></dl>

<dl class="first-deftypefn">
<dt class="deftypefn" id="index-_005f_005fbuiltin_005fsub_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_sub_overflow</strong> <code class="def-code-arguments">(<var class="var">type1</var> <var class="var">a</var>, <var class="var">type2</var> <var class="var">b</var>, <var class="var">type3</var> *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fsub_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fssub_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_ssub_overflow</strong> <code class="def-code-arguments">(int <var class="var">a</var>, int <var class="var">b</var>, int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fssub_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fssubl_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_ssubl_overflow</strong> <code class="def-code-arguments">(long int <var class="var">a</var>, long int <var class="var">b</var>, long int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fssubl_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fssubll_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_ssubll_overflow</strong> <code class="def-code-arguments">(long long int <var class="var">a</var>, long long int <var class="var">b</var>, long long int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fssubll_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fusub_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_usub_overflow</strong> <code class="def-code-arguments">(unsigned int <var class="var">a</var>, unsigned int <var class="var">b</var>, unsigned int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fusub_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fusubl_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_usubl_overflow</strong> <code class="def-code-arguments">(unsigned long int <var class="var">a</var>, unsigned long int <var class="var">b</var>, unsigned long int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fusubl_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fusubll_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_usubll_overflow</strong> <code class="def-code-arguments">(unsigned long long int <var class="var">a</var>, unsigned long long int <var class="var">b</var>, unsigned long long int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fusubll_005foverflow"> &para;</a></span></dt>
<dd>
<p>These built-in functions are similar to the add overflow checking built-in
functions above, except they perform subtraction, subtract the second argument
from the first one, instead of addition.
</p>
</dd></dl>

<dl class="first-deftypefn">
<dt class="deftypefn" id="index-_005f_005fbuiltin_005fmul_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_mul_overflow</strong> <code class="def-code-arguments">(<var class="var">type1</var> <var class="var">a</var>, <var class="var">type2</var> <var class="var">b</var>, <var class="var">type3</var> *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fmul_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fsmul_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_smul_overflow</strong> <code class="def-code-arguments">(int <var class="var">a</var>, int <var class="var">b</var>, int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fsmul_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fsmull_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_smull_overflow</strong> <code class="def-code-arguments">(long int <var class="var">a</var>, long int <var class="var">b</var>, long int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fsmull_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fsmulll_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_smulll_overflow</strong> <code class="def-code-arguments">(long long int <var class="var">a</var>, long long int <var class="var">b</var>, long long int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fsmulll_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fumul_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_umul_overflow</strong> <code class="def-code-arguments">(unsigned int <var class="var">a</var>, unsigned int <var class="var">b</var>, unsigned int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fumul_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fumull_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_umull_overflow</strong> <code class="def-code-arguments">(unsigned long int <var class="var">a</var>, unsigned long int <var class="var">b</var>, unsigned long int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fumull_005foverflow"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fumulll_005foverflow"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_umulll_overflow</strong> <code class="def-code-arguments">(unsigned long long int <var class="var">a</var>, unsigned long long int <var class="var">b</var>, unsigned long long int *<var class="var">res</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fumulll_005foverflow"> &para;</a></span></dt>
<dd>
<p>These built-in functions are similar to the add overflow checking built-in
functions above, except they perform multiplication, instead of addition.
</p>
</dd></dl>

<p>The following built-in functions allow checking if simple arithmetic operation
would overflow.
</p>
<dl class="first-deftypefn">
<dt class="deftypefn" id="index-_005f_005fbuiltin_005fadd_005foverflow_005fp"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_add_overflow_p</strong> <code class="def-code-arguments">(<var class="var">type1</var> <var class="var">a</var>, <var class="var">type2</var> <var class="var">b</var>, <var class="var">type3</var> <var class="var">c</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fadd_005foverflow_005fp"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fsub_005foverflow_005fp"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_sub_overflow_p</strong> <code class="def-code-arguments">(<var class="var">type1</var> <var class="var">a</var>, <var class="var">type2</var> <var class="var">b</var>, <var class="var">type3</var> <var class="var">c</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fsub_005foverflow_005fp"> &para;</a></span></dt>
<dt class="deftypefnx def-cmd-deftypefn" id="index-_005f_005fbuiltin_005fmul_005foverflow_005fp"><span class="category-def">Built-in Function: </span><span><code class="def-type">bool</code> <strong class="def-name">__builtin_mul_overflow_p</strong> <code class="def-code-arguments">(<var class="var">type1</var> <var class="var">a</var>, <var class="var">type2</var> <var class="var">b</var>, <var class="var">type3</var> <var class="var">c</var>)</code><a class="copiable-link" href="#index-_005f_005fbuiltin_005fmul_005foverflow_005fp"> &para;</a></span></dt>
<dd>
<p>These built-in functions are similar to <code class="code">__builtin_add_overflow</code>,
<code class="code">__builtin_sub_overflow</code>, or <code class="code">__builtin_mul_overflow</code>, except that
they don&rsquo;t store the result of the arithmetic operation anywhere and the
last argument is not a pointer, but some expression with integral type other
than enumerated or boolean type.
</p>
<p>The built-in functions promote the first two operands into infinite precision signed type
and perform addition on those promoted operands. The result is then
cast to the type of the third argument.  If the cast result is equal to the infinite
precision result, the built-in functions return <code class="code">false</code>, otherwise they return <code class="code">true</code>.
The value of the third argument is ignored, just the side effects in the third argument
are evaluated, and no integral argument promotions are performed on the last argument.
If the third argument is a bit-field, the type used for the result cast has the
precision and signedness of the given bit-field, rather than precision and signedness
of the underlying type.
</p>
<p>For example, the following macro can be used to portably check, at
compile-time, whether or not adding two constant integers will overflow,
and perform the addition only when it is known to be safe and not to trigger
a <samp class="option">-Woverflow</samp> warning.
</p>
<div class="example smallexample">
<pre class="example-preformatted">#define INT_ADD_OVERFLOW_P(a, b) \
   __builtin_add_overflow_p (a, b, (__typeof__ ((a) + (b))) 0)

enum {
    A = INT_MAX, B = 3,
    C = INT_ADD_OVERFLOW_P (A, B) ? 0 : A + B,
    D = __builtin_add_overflow_p (1, SCHAR_MAX, (signed char) 0)
};
</pre></div>

<p>The compiler will attempt to use hardware instructions to implement
these built-in functions where possible, like conditional jump on overflow
after addition, conditional jump on carry etc.
</p> 
</dd></dl>

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